1. Field of the Invention
The present invention relates to an improvement in an electrophotographic reproducing machine and, more particularly, to an improvement of an electrophotographic reproducing machine (referred to simply as "reproducing machine", hereinunder) of the type in which an electrostatic latent image is formed by projecting a document image on a charged photosensitive member and the latent image is developed by supplying a conductive or insulated developer to the surface of the photosensitive member by means of a developer carrying means capable of being applied with a biasing voltage.
2. Description of the Prior Art
FIG. 1 shows a known reproducing machine of the type mentioned above, in which a sleeve 2 as the developer carrying means of the developing device is held at the grounding potential, while FIG. 2 shows another known reproducing machine of the type in which the sleeve 2 is held at a constant potential. In the reproducing machine shown in FIG. 1, a relationship as shown in FIG. 3 is observed between the surface potential of the photosensitive member 3 and the amount of deposition of the developer, provided that the photosensitive member 3 is made of selenium. More specifically, the surface potential at which the developer starts to attach to the photosensitive member 3 is 50 to 100 V. Therefore, if the potential of the portion of the photosensitive member 3 corresponding to the white blank portion of the document exceeds above-mentioned potential, the developer undesirably attaches also to the background portion of the picture. This phenomenon is usually referred to as "fog" . In order to eliminate the fog, in the reproducing machine of the type shown in FIG. 2, a biasing voltage of several tens of volts of the same potential and polarity as the surface of the photosensitive member 3 is applied to the sleeve 2. The potential of the electrostatic latent image, including the portion corresponding to the white blank of the document, tends to fluctuate due to various reasons such as fatigue as a result of continuous reproduction, change in the ambient air temperature and so forth. In consequence, the generation of fog cannot be avoided perfectly even in the reproducing machine shown in FIG. 2. Such a reproducing machine also has been known as adapted to vary the level of the biasing voltage applied to the sleeve upon sensing the change of the surface potential of the photosensitive member 3. This improved reproducing machine, however, necessitates a sensor for sensing the surface potential of the photosensitive member 3 and a complicated circuit for controlling the biasing voltage.